T-dependent immune responses are characterized by the rapid appearance of IgM secreting plasma cells followed by the appearance of follicular germinal centers and isotype class switched, high affinity IgG, IgA, or IgE secreting plasma cells. Considerable recent attention has been given to the genes involved in specifying plasma cell development (BLIMP1) and germinal center development (BCL6 and AID, activation induced cytidine deaminase). Additionally, studies using model T-dependent antigens have provided insight into the qualitative and quantitative changes in B cells during an immune response. However, what continues to be lacking in our appreciation of activated B cell differentiation is an understanding of the signaling requirements that promote terminal differentiation into antibody secreting plasma cells or conversely, the signals that promote entry into the germinal center resulting in class switch recombination (CSR) and somatic hypermutation (SHM). The proposed work addresses the role of phosphatidyl inositol-3 kinase (PI3K) in directing the activated B cell fate decision toward plasma cell generation or the germinal center. Our preliminary data support the hypothesis that strong PI3K activity promotes plasma cell formation, whereas weak PI3K activity promotes CSR. We have shown that by reducing PI3K activity, chemically or genetically, we can promote CSR; which is inversely correlated with plasma cell generation but not proliferation. Our data suggest that this regulation operates at the level of controlling the expression and function of AID, and strongly implicate the Akt as the downstream target of PI3K activation that controls CSR. Further, we show that Akt-mediated inhibition of CSR occurs through the inactivation of the Foxo family of transcription factors. The specific aims of this proposal are designed to define the role of PI3K signaling in activated B cell fate decisions by i) characterizing in vivo plasma cell and germinal center development in B cell-specific PTEN-deficient animals, PTEN/CD19 double-deficient animals, and PTEN/BLIMP1 double-deficient animals, ii) assessing the impact of modulating PI3K signaling on genes involved in plasma cell differentiation, and iii) identifying PI3K responsive elements regulating AID expression and/or function. Relevance: The ability to form plasma cells and to undergo CSR is critical for antibody mediated immune responses. Hyper-IgM syndrome is a human immunodeficiency in which the ability to undergo CSR is defective resulting in a severe impairment in the ability to fight infections, whereas dysregulated plasma cell formation can ultimately lead to myeloma. Hence, understanding the pathways regulating these processes is a requisite step towards our ability to understand the etiology of these diseases. [unreadable] [unreadable] [unreadable] [unreadable]